1. Field of the Invention
This invention is in the field of electrolytic processing of materials and measurement of electrolyte parameters. More particularly, the present invention is in the field of methods for measuring characteristics of electrolytic processing, including, e.g., uniformity, or throwing power; electrochemical efficiency; and operating current density range.
2. Description of the Prior Art
Throwing power, electrochemical efficiency and the operating current density range are important characteristics of a processing electrolyte and, consequently, the process itself. They determine the uniformity of electrochemical action over articles of irregular shape--the distribution of plating thickness in the case of electrodeposition or the amount of metal removed in an anodic process. Throwing power is generally understood as the ability of an electrolyte to deposit metal cathodically in narrow recesses and small-diameter holes, or, conversely, to dissolve metal from shielded areas anodically. Electrochemical efficiency determines the actual amount of deposited or dissolved metal based on the amount of electricity used in the process. It often varies with the local current density and is a function of electrolyte composition. The variation of current density over the surface of an object undergoing electrochemical processing is known as the secondary current distribution; see Electroplating Engineering Handbook, ed. by L. Durney, Van Nostrand Reinhold Co., 1984, p. 461-469. Secondary current distribution is determined by polarization and other electrochemical effects modifying the primary current distribution, which is the ideal or theoretical current distribution in the absence of secondary effects. The operating current density range defines the upper and lower limits of current density for a given process, below and above which no acceptable results can be obtained.
The ability to quickly and accurately measure these parameters would provide the means for maintaining a processing electrolyte in optimum condition and evaluate effects that additives, impurities and changes in operating conditions have on the process performance.
Traditional methods for measuring throwing power, operating current density range and electrochemical efficiency of electrolytes are described in part by D. A. Luke in "Notes on Throwing Power Measurement", Transactions of the Institute of Metal Finishing, v. 61, 1983, p. 64-66, and Von J. Heyes in "Das elektrolytische Polieren von Stahl mit einem uberchlorsaurehaltigen Elektrolyten", Metalloberflache, N. 12, 1961, p. B181-B190. Such methods of the prior art require that a sample of electrolyte be tested in a plating cell such as the Haring or the Hull cell, both well known in the art.
To conduct a test such as those which utilize plating from a Haring or Hull cell, a sample of the solution is transferred to the appropriate cell, furnished with a power source, an anode and a cathode panel. Electrolysis is then carried out in the cell for a predetermined amount of time at the appropriate current level. Thereafter, the amount of deposited or dissolved metal (depending on the nature of the process) on different sections of the anode or the cathode panel is determined and used to calculate the throwing power, electrochemical efficiency and operating current density for the tested electrolyte.
There are no universally accepted units for the expression of throwing power. Often, the weight ratio of metal deposits formed at particular high and low current density areas of the cathode panel is used to express the throwing power of a plating bath.
In addition to their main disadvantage, i.e., the impossibility of measuring electrolyte properties directly in process tanks using existing counterelectrodes and current sources, traditional methods are cumbersome, time-consuming and, as pointed out by Luke, very sensitive to the electrolytic cell arrangement, especially the cathode-to-anode distance and relative position. Because of difficulties with the interpretation of test results, also discussed by Luke, traditional test methods have little predictive value. Consequently, the industry relies on a variety of tests involving the processing of an actual object of interest, such as, e.g. plating into a small-diameter hole (via) in a printed circuit board, and measuring the metal distribution over that object destructively to evaluate the throwing power of electrolytes. One such test was described by S. Shawki et al. in "Throwing Power", Metal Finishing, p. 59-61, December 1987. This approach is even more time-consuming and expensive than methods involving plating cells. Its use for routine process maintenance is, therefore, very limited.
More recently, R. F. Bernards et al. made an attempt to overcome difficulties associated with the described traditional approaches by proposing a method and apparatus for in-tank throwing power determination in U.S. Pat. No. 4,932,518 (1990). Their method involves two cathode assemblies, each consisting of one center, two edge and two surface electrodes, arranged parallel to each other and spaced apart in a plating tank. An additional thieving cathode is used to improve the plating uniformity over the parallel cathodes. A pair of anodes is suspended perpendicular to the cathode assemblies on both sides of them, with the assemblies centered between the two anodes. Throwing power in this method is expressed as the current ratio between the edge and center electrode portions of the cathode assemblies during electroplating.
This method, while allowing for tests to be carried out directly in a plating tank, has nevertheless not become a widely used or accepted testing procedure. Test results are, as in the previously described classical methods, dependent on the anode-to-cathode distance and relative position. Moreover, the distance between the two halves of the cathode assembly has an additional bearing on the results. To express throwing power as the current ratio is to assume that electrochemical efficiency is independent of the current density, which in general is not true. Indeed, it is well known in the art that many electrochemical systems rely on reduced efficiency at higher current densities (caused by polarization and other effects) for improved throwing power.
Solution agitation is an important process variable, but neither of the methods discussed above is capable of measuring its effects on throwing power quantitatively. Agitation, which has a strong influence on the processing uniformity, can only be quantified if provisions are made for accurate solution circulation rate measurement. None of the discussed methods makes such provisions or can be easily adopted for that purpose.
Summarizing, it can be said that the best known to date method for in-tank measurement of throwing power according to U.S. Pat. No. 4,932,518 is very complicated, requiring a total of eleven cathode and two anode electrodes to be arranged a certain way in a plating tank, sensitive to the anode-to-cathode distance and relative position, and limited in scope. Only allowing measurement of what is known in the art as the secondary current distribution for a particular cell arrangement, the method taught in the U.S. Pat. No. 4,932,518 does not allow determination of the true electrolyte processing uniformity. The method can not be used to determine the electrochemical efficiency of a process, without which throwing power can not be established based on the current ratio alone. Further, that method does not provide quantitative assessment of agitation effects on throwing power, nor does it predict the processing uniformity for actual objects, such as, e.g., blind vias in printed circuit boards.